1. Field of the Invention
The present invention relates generally to lithographic printing presses and more particularly it concerns an apparatus for eliminating excess dampening solution trapped in ink found on rollers of a lithographic printing press.
2. Description of the Prior Art
During offset printing it is not uncommon to develop a build-up of excess dampening solution in the ink on the rollers. Its occurrence is inherent to the printing process. A variety of printing problems are the result. How the build-up occurs and what efforts have been made to eliminate/reduce it are described below.
Each printing head of an off-set lithographic printing press consists of several basic components and assemblies: an ink train, a dampening system, a printing plate and plate cylinder, a blanket and blanket cylinder, and an impression cylinder. These components and assemblies cooperate to lay the proper ink image and ink film thickness on the sheet or web.
The ink train transfers ink to the plate. Ink trains are ordinarily made up of an ink fountain for storing a supply of ink; a slowly rotating ink-fountain roller for metering ink from the ink fountain to the remainder of the ink train; a series of soft and hard rollers running at press speed for milling and smoothing the ink; and an ink-ductor roller for transferring ink from the ink-fountain roller to the first roller in the series of soft and hard rollers. Inking form rollers transfer the ink from the series of rollers to the plate.
The typical dampening system consists of a dampening-solution fountain, a dampener-fountain roller, and a series of rollers that transfer a controlled amount of dampening solution to a dampening form roller. The dampening form roller transfers the solution from the series of rollers to the plate.
The plate cylinder carries the printing plate. On the printing plate are ink receptive image areas and dampening solution receptive non-image areas. The lithographic printing plate is planographic. This means the image areas and non-image areas are essentially on the same plane.
In order to properly ink the image areas of the plate the dampening system usually first wets the plate's non-image areas so that the ink being transferred from the inking form rollers to the plate will adhere to the image areas only. It is a feature of printing inks to take up a certain amount of dampening solution to facilitate this process. For this reason some dampener systems are also linked to the ink train via a bridging roller. These systems are said to be partially integrated. Some dampener systems do not have a dampener form roller at all but feed dampening solution directly to the first ink form roller. These fully-integrated dampeners depend wholly on the dampening solution carrying ability of the ink for plate dampening.
The inked image on the plate is transferred to a rubber blanket attached to the outer diameter of the blanket cylinder. The image is then transferred to a paper sheet or web that passes between the blanket and impression cylinders. A characteristic of rollers carrying fluid is the `splitting` of the fluid as it passes between contacting rollers. Fluid splitting means that as two rollers are pressed together and rotated, a percentage of fluid found on the first roller, the fluid-transferring roller, will be passed to the second roller, the fluid-receiving roller. The remaining percentage of fluid is retained on the transferring roller. The larger the percentage of fluid transfer, the greater the ability of the roller system to transfer fluid. Under the proper conditions, all rotating fluid-carrying rollers in contact function this way. The direction of fluid flow generally determines which roller is the transferring roller and which roller is the receiving roller. The degree to which a fluid splits, or rate of flow, depends on several factors; including fluid source feed rate, transferring roller to receiving roller orientation, roller train orientation, roller covering material, roller diameters and durometers, and fluid characteristics. Printing presses are designed to cause ink and dampening solution to flow toward the plate cylinder.
To a certain extent, fluid splitting works in both directions--from transferring roller to receiving roller and vice versa. In a printing press, for example, we can see that while the dampener form roller is feeding dampening solution to the non-image areas of the printing plate a small amount of ink is picked up by the dampener form roller from the image areas of the plate. Likewise, as the ink form rollers are feeding ink to the image areas on the priming plate, a small amount of dampening solution is picked up by the ink form rollers from the non-image areas of the plate. Because printing presses are designed to cause the ink and dampening solution to flow toward the plate cylinder, the effect of reverse splitting is kept to a minimum; though not eliminated.
As noted above, inks are designed to take up a certain percentage of dampening solution. This facilitates ink lay-down by further wetting the non-image areas of the plate. However, too much dampening solution can be forced into the ink in the inking system either by the inking form rollers picking up fluid from the plate by reverse splitting or, if the press has a partially of fully integrated dampening system, directly through the bridging roller. In addition, because of excessive dampening solution feed, ink accumulated on the dampener form roller can be forced to take on too much dampening solution.
When an excessive amount of dampening solution is picked up by ink it becomes water logged and breaks down. This is commonly called over-emulsification. The results are extreme ghosting and loss of color density in the print, mottled print, ink piling on the inked rollers, and sheet curl resulting in misregistration and feed problems. To eliminate the problems, the press operator will usually have to clean the entire ink train and dampener and then replenish the ink and dampening solution fountains. This work stoppage results in product delivery delays and measurable economic loss.
Attempts have been made to prevent excess dampening solution buildup and/or remove the excessive dampening solution from over emulsified ink. One method is to install an air bar that directs a stream of air against the surface of one or several of the inked rollers to force evaporation of excess solution. Two inventions that exemplify this method are disclosed in U.S. Pat. No. 4,524,689 entitled "DEHYDRATION APPARATUS FOR PRINTING PRESS INKING SYSTEM" and U.S. Pat. No. 5,085,142 entitled "DAMPENING FLUID EVAPORATOR." The method is limited in that it tends to evaporate only the accumulation of excessive dampening solution occurring on the surface of the ink coated rollers. Lacking in the art is an effective fluid evaporating system that could direct a stream of air against the exterior surface of an inked roller while simultaneously working the ink so to more efficiently remove excess dampening solution - not only on the surface of the ink film but also that which is mixed into the ink film.